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    Toxicological evaluation of certain veterinary drug
    residues in food



    WHO FOOD ADDITIVES SERIES 39





    Prepared by:
    The forty-eighth meeting of the Joint FAO/WHO Expert
    Committee on Food Additives (JECFA)



    World Health Organization, Geneva 1997


    ENROFLOXACIN (addendum)

    First draft prepared by Dr P.L. Chamberlain
    Center for Veterinary Medicine
    Food and Drug Administration,
    Rockville, Maryland, USA

    1. Explanation
    2. Biological data
       2.1 Biochemical aspects
           2.1.1  Absorption, distribution, excretion
       2.2 Toxicological studies
           2.2.1  Special studies on microbiological effects
                  2.2.1.1 Tests  in vitro
                  2.2.1.2 Observations in humans
    3. Comments
    4. Evaluation
    5. References


    1.  EXPLANATION

         Enrofloxacin is a fluroquinolone antibiotic that was evaluated at
    the forty-third meeting of the Committee (Annex 1, reference 114). A
    temporary ADI of 0-0.6 g/kg bw was established at that time on the
    basis of limited microbiological data. The Committee requested the
    following information for evaluation in 1997: (i) detailed reports of
    the investigations of MIC values  in vitro that were submitted for
    evaluation and (ii) information on the effects of enrofloxacin and
    ciprofloxacin on specific genera of microorganisms obtained from the
    human intestine. In addition, the Committee required that the results
    of studies to determine the antimicrobial activity of the residues
    other than enrofloxacin and ciprofloxacin be submitted for review as
    soon as they became available.

         This monograph addendum summarizes new data on the
    microbiological activity of enrofloxacin and ciprofloxacin and the
    pharmacokinetic properties of ciprofloxacin in humans that have become
    available since the previous evaluation (Annex 1, reference 113).

    2.  BIOLOGICAL DATA

    2.1  Biochemical aspects

    2.1.1  Absorption, distribution and excretion

         Ciprofloxacin has been identified as a major metabolite of
    enrofloxacin in some food-producing animals. Because it is used
    routinely in human medicine, its pharmacokinetic properties in humans
    have been well studied.

         The time to peak serum concentration in healthy subjects given a
    single oral dose of 400 mg ciprofloxacin is 1-2 h. The peak serum
    concentration is reported to be 1.5 g/ml. The beta half-life of
    elimination is 3.5 h. The area under the curve for serum concentration
    versus time is about 5.5 mg.h/litre. The absolute oral bioavailability
    is 63-69%. Administration with food does not cause clinically
    important alterations in oral bioavailability. Elimination occurs by
    both renal and non-renal routes, and metabolism accounts for < 20% of
    elimination. The apparent volume of distribution exceeds 70 litres
    (total body water). Only 14-25% of ciprofloxacin binds to serum
    proteins, which facilitates tissue penetration and access to target
    sites. The peak concentrations in urine are 25 to many hundred times
    the serum concentrations. The concentrations in renal tissue tend to
    be two to 10 times higher than the serum concentrations. The faecal
    concentrations in the gastrointestinal tract are very high, and the
    concentrations in bile tend to be two to 10 times greater than the
    serum concentrations. The peak concentrations in saliva and bronchial
    secretions are usually 0.2-1.0 of the serum concentrations, but the
    concentrations in lung tissue are 1.6 to four times greater.

         The main sites of absorption of ciprofloxacin are the duodenum
    and jejunum. Radiolabelled ciprofloxacin given intravenously to rats
    is found in the lower gastro-intestinal tract too soon after injection
    to be accounted for by biliary excretion, suggesting transintestinal
    secretion; 11% of ciprofloxacin administered intravenously to healthy
    subjects was secreted across the bowel wall into the intestinal lumen.
    Transintestinal elimination has been suggested elsewhere as the route
    of elimination by the gastrointestinal tract of humans (Wolfson &
    Hooper, 1991).

    2.2  Toxicological studies

    2.2.1  Special studies on microbiological effects

    2.2.1.1  Tests  in vitro

         The minimum concentration of enrofloxacin that inhibited 50%
    (MIC50) of colonies of 100 bacterial strains (10 isolates of 10
    genera) of human intestinal origin was determined at three inoculum
    levels: 109, 107, and 105 colony forming units (cfu) per ml.
    Isolates were obtained before antibiotic therapy. The spectrum of
    bacteria included gram-positive and gram-negative organisms and

    facultative, moderate, and stringent anaerobes. This spectrum was
    considered to be representative of the typical overall human
    intestinal flora with respect to oxygen tolerance and taxonomy. The
    study was conducted in full accordance with the principles of GLP. The
    results are presented in Table 1 (Marshall, 1996).

        Table 1. Mean minimum concentrations of enrofloxacin resulting in
    inhibition of 50% of colonies (MIC50) of 100 bacterial strains of
    human intestinal origin at three inoculum levels

                                                                          

    Genus                      No. of strains      Mean MIC50 (g/ml)
                                                                          
                                                 109      107      105
                                                                          

    Escherichia coli                10           0.062    0.031    0.031
    Enterococcus spp.               10           1        1        0.25
    Lactobacillus spp.              10           2a       0.5      0.5
    Proteus vulgaris                10           0.25     0.125    0.125
    Bacteroides spp.                10           2        1        2
    Bifidobacterium spp.            10           2b       0.5      0.125
    Fusobacterium spp.              10           0.25     0.125    0.125
    Eubacterium spp.                10           0.5      0.25     0.25
    Peptostreptococcus spp.         10           0.5b     0.25     0.25
    Clostridium spp.                10           4        0.5      0.062
                                                                          

    a Nominal concentration of 109 not achieved for three species
    b Nominal concentration of 109 not achieved for one specie
    
         In another study  in vitro, the MIC50 levels of enrofloxacin
    and nine of its metabolites were tested against 164 aerobic bacterial
    strains of human origin (23 isolates of each of five genera) at a
    single inoculum level (1  104 cfu/inoculation site, except for
     Proteus spp. which were inoculated at 1  105 cfu/inoculation
    well). Strains of the genera  Escherichia, Enterococcus, 
     Staphylococcus, and  Proteus were isolated from clinical sources
    during the period 1992-95. The strains of  E. coli, Staphylococcus, 
    and  Proteus tested in this study were selected after classification
    as 'untreated' with fluoroquinolones. Because of their low natural
    susceptibility to fluoroquinolones, strains of the genera
     Enterococcus and  Lactobacillus were not selected. Lactobacilli
    were isolated from the oral cavities of healthy people who had not
    received antibiotics within four weeks before sampling.

         The metabolites of enrofloxacin tested were ciprofloxacin,
    oxoenrofloxacin, oxociprofloxacin,  N-formyl ciprofloxacin,
    desethylene enrofloxacin, desethylene ciprofloxacin, ring-opened
    oxociprofloxacin, 7-aminoacetic fluoroquinolonic acid, and 
    7-aminofluoroquinolonic acid. The study was conducted in full
    accordance with the Standard Laboratory Protocol, the principles of
    the National Committee of Clinical Laboratory Standard, and the
    guidelines of the Deutsches Institut fur Normung e.V. The
    susceptibility of the tested strains to enrofloxacin and its
    metabolites is summarized in Table 2. The most active substances
    tested were enrofloxacin and ciprofloxacin, and the most susceptible
    bacterial species was  E. coli (Pirro, 1996).

         The MIC50 of ciprofloxacin was determined against 100 bacterial
    strains of human intestinal origin (10 isolates of each of 10 genera)
    at a single inoculum level (107 cfu/ml). The isolates were obtained
    from clinical cases before the initiation of antibiotic therapy. The
    spectrum of bacteria included gram-positive and gram-negative
    organisms and facultative, moderate, and stringent anaerobes. This
    spectrum was considered to be representative of the typical overall
    human gut flora with respect to oxygen tolerance and taxonomy. The
    study was conducted in accordance with the principles of GLP
    (Pridmore, 1996a). The results are given in Table 3.

         In another study, the effect of pH on the MIC50 of enrofloxacin
    against 36 bacterial isolates of human intestinal origin  in vitro 
    was evaluated. The pH levels were chosen to encompass the range of
    conditions likely to occur in the lower regions of the human
    intestinal tract, while avoiding extremes likely to inhibit bacterial
    growth  in vitro. The isolates were obtained from human clinical
    cases before initiation of antibiotic therapy. The spectrum of
    bacteria included Gram-positive and gram-negative organisms and
    facultative, moderate, and stringent anaerobes. This spectrum was
    considered to be representative of the overall typical human
    intestinal flora with respect to oxygen tolerance and taxonomy. The
    strains were considered as representative of their respective genera
    if they exhibited MIC50 values against enrofloxacin that were in the
    region of the geometric mean for their genus in the study of Marshall
    (1996), summarized above. A larger number of  E. coli isolates was
    selected on the basis of reference data generated by the sponsor. A
    0.5 McFarland standardized inoculum was produced from each culture;
    however, the inocula were not enumerated. The study was conducted in
    full accordance with the principles of GLP. The results of this study
    are presented in Table 4. The investigator reported that reduction of
    fluoroquinolone activity at low pH has been demonstrated by other
    investigators (Pridmore, 1996b).


        Table 2. Susceptibility of aerobic human bacterial strains to enrofloxacin and nine of its metabolites

                                                                                                             

    Compound                 Mean MIC50(g/ml) against:
                                                                                                             
                             E. coli        Proteus        Lactobacillus    Enterococcus     Staphylococcus
                             (33 strains)   (49 strains)   (32 strains)     (23 strains)     (25 strains)
                                                                                                             

    Enrofloxacin              0.03           0.125              8                 1            0.125
    Ciprofloxacin             0.015          0.03              16                 1            0.25
    Oxoxiprofloxacin          0.25           1                  4                 2            0.25
    Desethylene               2              4              > 128             > 128           32
       ciprofloxacin
    Ring-opened              16             32              > 128             > 128           16
       oxociprofloxacin
    7-Aminoacetic            16              8              > 128             > 128           32
       fluoroquinolonic
       acid
    Desethylene               2              8              > 128                16
       enrofloxacin
    N-Formyl cipro-           0.06           0.125             16                 2            0.25
       floxacin
    7-Aminofluoro-            0.5            0.25           > 128               128            8
       quinolonic acid
    Oxoenrofloxacin          Not tested      1                  8                 2            0.125
                                                                                                             
    
    Table 3. Mean minimum concentrations of
    ciprofloxacin resulting in inhibition of 50% of
    colonies (MIC50) of 10 strains of each of 10
    bacteria of human intestinal origin at one
    inoculum level of 107 cfu/ml

                                                      
    Genus                         Mean MIC50
                                  (g/ml)
                                                      
    Escherichia coli              0.016
    Enterococcus spp.             1
    Lactobacillus spp.            0.5
    Proteus vulgaris              0.031
    Bacteroides spp.              8
    Bifidobacterium spp.          0.031
    Fusobacterium spp.            0.125
    Eubacterium spp.              0.5
    Peptostreptococcus spp.       0.125
    Clostridium spp.              0.25a
                                                      

    a This result is questionable owing to low
    inoculum counts and ranges (103-107 cfu/ml)
    for the strains tested.

        Table 4. Minimum inhibitory concentrations of enrofloxacin or concentrations that 
             inhibit 50% of colonies of bacterial isolates of human intestinal origin 
             in culture at three pH levels

                                                                                         
    Genus                         No. of      Mean MIC50 (g/ml)a or
                                  isolates    geometric mean MIC (g/ml)
                                                                                         
                                              pH 7        pH 6          pH 5.2
                                                                                         
    Escherichia coli              9           0.031       0.062         0.25
    Proteus spp.                  3           0.125       0.315         2.52
    Lactobacillus spp.            3           0.400       0.500         2.50
    Enterococcus spp.             3           0.63        1.00          4.00
    Bacteroides spp.              3           1.26        2.00          Not calculatedb
    Fusobacterium spp.            3           0.315       0.315         Not calculatedb
    Peptostreptococcus spp.       3           0.198       0.250         0.200
    Clostridium spp.              3           0.157       0.198         Not calculatedb
    Eubacterium spp.              3           0.198       0.198         0.125
    Bifidobacterium spp.          3           0.397       1.00          5.04
                                                                                         

    a  Mean MIC50 calculated only for E. coli, because of small numbers of isolates of 
       other genera tested
    b  Insufficient growth in the three isolates
    
         The effects of differences  in vitro and  in vivo on the
    microbiological activity of enrofloxacin against selected bacterial
    strains of human intestinal origin were investigated using a test
    system in which enrofloxacin and the test strains were exposed to
    conditions simulating those found in the human intestinal tract. The
    study was conducted in full compliance with the principles of GLP. The
    MIC50 values of enrofloxacin against the cultures used in this study
    had been determined previously in the study of Marshall (1996). Two
    isolates of each of five genera obtained from patients before
    antibiotic therapy were selected. The spectrum of bacteria included
    gram-positive and gram-negative organisms and facultative, moderate,
    and stringent anaerobes. This spectrum was considered to be
    representative of the overall typical human intestinal flora with
    respect to oxygen tolerance and taxonomy. The procedure used was as
    follows:

    *    Enrofloxacin was added at two concentrations to cooked meat
         medium:

         -    at a concentration similar to the geometric mean MIC50
              determined for the respective microbial groups and

         -    at a concentration likely to be achieved in the gut after
              consumption of typical residue levels.

    *    The enrofloxacin/cooked meat medium mixture was allowed to
         equilibrate for 1 h, then supplemented with physiological levels
         of pepsin, adjusted to pH 2, and incubated anaerobically at 37C
         for 1 h. The mixture was supple-mented with physiological levels
         of bile salts and pancreatin, then adjusted to pH 7.

    *    The culture was inoculated into the mixture and incubated at 37C
         for 18 h.

    *    Viable counts were performed to determine bacterial survival, and
         the results were compared with those for a control medium
         inoculated with the test culture but no bacteria.

    The results of this study are summarized in Table 5. The investigator
    pointed out that the viable counts of all 10 bacterial strains exposed
    to enrofloxacin in this intestinal model experiment increased during
    the 18-h incubation period. These results suggest that the
    concentrations of enrofloxacin tested, i.e. those that could
    potentially occur in the human intestine, are unlikely to have a
    significant effect on the complex intestinal ecosystem of the human
    intestine (Watson, 1996).


        Table 5. Effect of enrofloxacin on human intestinal isolates after incubation in a simple intestinal model in vitro

                                                                                                                     
    Strain                      Enrofloxacin       Inoculum             Total viable count             Enrofloxacin
                                concentration      density             (cfu/ml test system)            MIC (g/ml)
                                (g/ml)            (cfu/ml)                                      
                                                                    On inoculation   After 18 h
                                                                                                                     
    Enterococcus faecalis       0                  1.6  109        4.9  106        1.3  108         1
                                0.56                                5.6  106        5.6  107
                                0.90                                6.4  106        6.1  107
    Enterococcus spp.           0                  3.1  108        1.3  106        9.4  107         1
                                0.56                                1.2  106        6.1  107
                                0.90                                1.4  106        6.9  107
    Escherichia coli            0                  9.1  108        7.0  106        9.6  107         0.062
                                0.56                                7.4  106        5.2  107
                                0.04                                8.9  106        8.0  107

    Escherichia coli            0                  2.3x 109         4.7  106        8.9  107         0.062
                                0.56                                3.9  106        2.9  107
                                0.04                                5.0  106        7.5  107
    Clostridium                 0                  1.1  108        1.3  106        > 1.0  108       2
    sporogenes                  0.56                                1.5  106        > 1.0  108
                                0.90                                1.2  106        > 1.0  108
    Clostridium                 0                  7.1  108        4.1  106        > 1.0  108       0.25
    perfringens                 0.56                                3.8  106        > 1.0  108
                                0.90                                4.7  106        > 1.0  108
    Bifidobacterium             0                  2.7  107        8.9  103        2.1  105         0.25
    adolescentis                0.56                                5.4  102        5.1  104
                                0.4                                 3.0  102        5.5  104
    Bifidobacterium spp.        0                  7.8  109        6.0  104        2.0  105         0.5
                                0.56                                5.8  104        2.6  105
                                0.4                                 1.1  105        2.9  105
    Bacteroides                 0                  4.4  106        1.7  104        1.1  109         2
    thetaiotaomicron            0.56                                1.7  104        1.6  109
                                1.4                                 1.8  104        1.6  109
    Bacteroides vulgatus        0                  6.9  106        7.8  104        1.7  109         2
                                0.56                                7.6  104        1.8  109
                                1.4                                 6.9  104        1.6  109
                                                                                                                     
    
    2.2.1.2  Observations in humans

         Fluroquinolones as a class have a broad spectrum of activity
    against aerobic gram-negative bacteria, and their primary human
    clinical use is for selective decontamination of the gastrointestinal
    tract by decreasing the intestinal reservoir of potential aerobic and
    facultative anaerobic pathogens while preserving the predominant
    anaerobic bacteral flora. These conditions are clinically useful in
    the treatment of travellers' diarrhoea, therapy for immunocompromised
    patients, selective decontamination before colorectal surgery, and
    therapy for burn victims and leukaemia patients. Therapeutic oral
    doses of fluroquinolones to humans do not alter the intestinal
    bacterial ecology or weaken the barrier effect. In addition, anaerobic
    bacteria such as  Bifidobacterium, Bacteroides, Eubacterium, 
     Fusobacterium, and  Peptostreptococcus spp. are the main components
    of the human gastrointestinal tract and are affected only slightly or
    not at all by fluroquinolones. Although  E. coli is extremely
    sensitive to fluroquinolones in general, this species is a minor
    component of the flora in the gastrointestinal tract (Goldstein 
     et al., 1987; Midtvedt, 1990; Vollaard  et al., 1990; Lewin 
     et al., 1991; Boisseau, 1993; Nord, 1993; Hill, 1995; Nord, 1995).

         A study of the effects of ciprofloxacin on human intestinal flora
     in vivo was conducted in 12 healthy male subjects aged 19-40 years,
    who were given an oral dose of 500 mg of ciprofloxacin every 12 h for
    seven days. Faecal specimens were collected on the day before the
    first dose was taken, on the last day of treatment, and one week
    later. Bacterial counts were made, and ciprofloxacin concentrations
    were assayed by a microbiological assay (sensitivity not stated). No
    antibacterial activity was found in specimens taken before treatment
    or on day 14. On day 7, the concentration of ciprofloxacin in the
    faeces was 185-2220 g/g (mean, 891; SD, 624). The authors attributed
    these high concentrations to poor gastrointestinal absorption and not
    to biliary excretion. They remarked that, although biliary excretion
    is known to be significant in humans, it cannot account for the amount
    of drug that is recovered from faeces (unpublished data). The effects
    of ciprofloxacin on the faecal flora of the volunteers is shown in
    Table 6. In addition, although most (75%) of the anaerobic strains
    were sensitive to ciprofloxacin before treatment, only 12% were
    sensitive after treatment. This difference was statistically
    significant (p < 0.001) (Brumfitt  et al., 1984).

         The effects of therapeutic oral doses of ciprofloxacin on human
    intestinal flora were studied in 12 patients with acute leukaemia who
    were given a prophylactic oral dose of 500 mg ciprofloxacin every 12 h
    during treatment to induce remission. The mean duration of treatment
    was 42 days. The colon enterobacteria were eliminated within three to
    five days.  Bacteroides and  Clostridium species were not affected,
    but the numbers of anaerobic cocci were decreased.  Pseudomonas and
     Acinetobacter species were recovered, but these did not colonize or
    cause infection (Rosenberg-Arska  et al., 1985).

    Table 6. Mean log10 counts of faecal flora per gram of
    faeces of 12 healthy male volunteers treated orally with
    ciprofloxacin

                                                                     

    Flora                    Pretreatment     Day 7       Day 14
                                                                     

    Coliforms                7.24             < 2         6.25
    Faecal streptococci      6.3              3.31a       6.93
    Other streptococcib      5.61             2.72a       4.22
    Staphylococci            4.86             2.14a       3.07a
    Yeasts                   2.75             3.45        3.27
    Anaerobes                9.64             8.54        9.49
                                                                     

    a Difference statistically significant from pretreatment value
      (p < 0.05)
    b Usually Lancefield group C


         The effects of therapeutic oral doses of ciprofloxacin on human
    intestinal flora were also studied in 12 volunteers (six male, six
    female, 21-30 years old) given 400 mg ciprofloxacin orally every 12 h
    for seven days. Faecal samples were obtained one to two days before
    and two, five, eight, 10, and 15 days after ingestion of
    ciprofloxacin. Qualitative and quantitative analysis of microflora
    were performed. The results are shown in Table 7. The authors noted
    that anaerobic species that are considered to be the main stabilizing
    components of the bowel flora ( Bacteroides and  Bifidus species)
    were not affected by treatment; furthermore, no selection for 
     C. difficile was observed. This study thus shows that treatment with
    ciprofloxacin can eliminate major components of the aerobic 
    gram-negative flora with little or no influence on anaerobic organisms
    (Enzensberger  et al.,  1985).

         The effect of repeated oral doses of ciprofloxacin (500 mg every
    12 h for five consecutive days) on the oropharyngeal and colon
    microflora was investigated in 12 healthy volunteers. The peak serum
    concentration of ciprofloxacin was reported to be 2.8 mg/litre. Of the
    oral microflora, only the aerobic gram-negative cocci, e.g. 
     Neisseria  spp., were affected. In the colon, marked decreases in
    the numbers of enterobacteria and enterococci were observed; changes
    in the anaerobic microflora of the colon were considered minor. The
    oral and colonic microflora returned to normal 14 days after the last
    treatment. No new colonization of ciprofloxacin-resistant bacteria
    (MIC50 > 1 mg/l) was observed. Neither  C. difficile nor its
    cytotoxin was detected (Bergan  et al., 1986).


        Table 7. Effects of oral doses of ciprofloxacin on faecal flora (mean colony forming units per gram 
             faeces (log10))

                                                                                                        

    Species                   Day of treatment
                                                                                                        
                              0           2           5           8            10           15
                                                                                                        

    Escherichia coli          7.6         0           0           0            5.8 (2)      6.8 (8)
    Proteus vulgaris          0           0           0           0            4.0 (1)      7.6 (1)
    Providencia stuartii      0           0           0           0            7.6 (1)      0
    Citrobacter freundii      0           0           6.9 (1)     5.3 (1)      8.5 (1)      5.4 (3)
    Streptococcus faecalis    7.8         5.7 (9)     5.5 (7)     5.8 (8)      6.2 (7)      6.1 (9)
    Streptococcus faecium     7.4 (4)     5.7 (3)     5.3 (3)     4.2 (5)      6.2 (6)      6.1 (10)
    Bacteroides spp.          9.3         8.7         9.2         9.5          9.5          9.8
    Bifidobacterium spp.      9.1         8.3         8.6         9.3          9.6          9.1
    Candida spp.              5.3 (3)     4.1 (6)     4.6 (6)     5.6 (5)      5.0 (5)      4.9 (2)
    Geotrichum spp.           6.6 (1)     4.5 (4)     4.3 (4)     3.6 (1)      4.8 (4)      4.3 (2)
                                                                                                        

    In parentheses, number of volunteers if fewer than 12
    


         The effect of ciprofloxacin on the colonization resistance of
    colonic microflora was investigated in 12 healthy volunteers given 
    50 mg ciprofloxacin every 6 h for six days. After two or three days of
    treatment, Enterobacteriaceae strains had been eliminated from faecal
    samples of all volunteers. A slight effect was observed on
    enterococci, and the number of  Candida spp. showed a minor increase.
    No ciprofloxacin-resistant bacteria were isolated. The flora was
    considered normal one week after the last treatment (van Saene 
     et al., 1986).

         Six volunteers were given 500 mg ciprofloxacin orally once a day
    for five days. Faecal samples were collected before, during, and after
    treatment and analysed qualitatively and quantitatively for aerobic
    and anaerobic microorganisms. In all volunteers, a marked reduction in
    Enterobacteriaceae strains was observed during the treatment period
    (from 106-107 to < 104 cfu/g faeces). In two volunteers,
    colonization by resistant coagulase-negative staphylococci or
     Corynebacterium spp. was observed. These strains were not detected
    by the eighth day of the study. In one volunteer, the anaerobic
    bacteria count decreased from 1012 to 109 cfu/g faeces during the
    study (Holt  et al., 1986).

         Fourteen patients with liver cirrhosis were treated for urinary
    or respiratory tract infections with 250 mg ciprofloxacin twice daily
    or 500 mg ciprofloxacin once daily for five to 10 days. A marked
    decrease in the Enterobacteriaceae count was observed during the first
    few days at both doses; they disappeared between days 3 and 6 of
    therapy but returned to normal two weeks after the last treatment. The
    concentrations of  Bacteroides spp. decreased during therapy. Two
    patients, one receiving 250 mg and the other receiving 500 mg, had
     Candida albicans in their faeces during therapy, and these were
    still detectable 14 days after the last dose (Esposito  et al., 
    1987).

         Studies of the effect of fluoroquinolones on the intestinal
    microflora have been reviewed, including a summary of the results of
    studies with ciprofloxacin, as shown in Table 8 (Korten & Murray,
    1993).

         Ten healthy volunteers were given single oral doses of 750 mg
    ciprofloxacin, and blood samples were collected 1 h later. Faecal
    samples were collected once before treatment then daily after
    treatment for eight consecutive days. The mean serum concentration of
    ciprofloxacin in the 10 volunteers 1 h after administration of the
    drug was about 3 mg/litre. Faecal concentrations exceeded 3 mg/g in at
    least one sample from each volunteer on days 2-5 after treatment. The
    author did not propose an explanation for the sustained high
    concentrations of the drug in faeces but attributed them to failure of
    complete absorption and to gastrointestinal secretion of
    ciprofloxacin. The total counts of anaerobes, streptococci,
    staphylococci, and yeasts were not significantly modified by
    treatment. No overgrowth by  P. aeruginosa and no colonization by 

     C. difficile was observed, except in one subject who was apparently
    a healthy carrier of  C. difficile. A 2 log10 decrease in total
    aerobe counts was attributed to a significant reduction in the total
    count of Enterobacteriaceae (8 log10 cfu/g faeces down to 6 log10
    cfu/g faeces; p < 0.01). A slight but significant increase in the
    counts of Enterobacteriaceae resistant to nalidixic acid was also
    observed (from 2 to 4 cfu/g faeces; p < 0.01). This resistance showed
    a return to pretreatment levels on days 5-8 after treatment (Pecquet
     et al.,  1990).


        Table 8. Representative studies of the effect of ciprofloxacin on human intestinal flora

                                                                                                                                               

    Dose     No. of      Length of    No. of     Effect  on                                     Colonization   Emergence       Reference
    (mg)     doses/day   treatment    patients                                                  with yeasts    of resistance
                         (days)                  Enterobacteria   Enterococci  Anaerobes
                                                                                                                                               

    500      2           7            12         +++              ++           -                -              + (anaerobes)   Brumfitt et al.
                                                                                                                               (1984)
    400      2           7            12         +++              +            -                -              -               Enzensberger 
                                                                                                                               et al. (1985)
    500      2           42           15         +++              +            +                -              -               Rosenberg-Arska
                                                                                                                               et al. (1985)
    500      2           5            12         +++              ++           +                -              -               Bergan et al.
                                                                                                                               (1986)
    500      1           5            6          +++              +            +                -              -               Holt et al.
                                                                                                                               (1986)
    250      2           5-10         7          +++              -            +                Minor          -               Esposito et al. 
                                                                                                                               (1987)
    500      1           5-10         7          +++              -            +                Minor          -
    500      2           10           23         +++              ND           ND               -              -               Daikos et al. 
                                                                                                                               (1987)
    50       2a                                                                                                                de Vries-Hospers
                                                                                                                               et al. (1987)
    100      2           5            10         +++              +            -                Yesb           -
    200      2
    500      2           10           12         +++              -            ++(Veillonella)  -              -               Shah et al. 
                                                                                                                               (1987)
    500      2           1-42         11         +++              -            ND               -              -               Scully et al. 
                                                                                                                               (1987)
    /750     3
    50       4           7            15         +++              +            -                Yesb           ND              van Saene 
                                                                                                                               et al. (1988a)
    200      1           6            6          +++              +            -                Minor          -               van Saene 
                                                                                                                               et al. (1988b)
                                                                                                                                               

    Table 8. (continued)

                                                                                                                                               

    Dose     No. of      Length of    No. of     Effect  on                                     Colonization   Emergence       Reference
    (mg)     doses/day   treatment    patients                                                  with yeasts    of resistance
                         (days)                  Enterobacteria   Enterococci  Anaerobes
                                                                                                                                               


    500        1         7            25         +++              -            ND               +c             -               Rademaker 
                                                                                                                               et al. (1989)
    750        2         2            21         +++              ++           +++              -              -               Brismar et al. 
                                                                                                                               (1990)
    /400 i.v.  2
    500        2         5            14         ++               ++           +                -              ND              Ljungberg 
                                                                                                                               et al. (1990)
                                                                                                                                               

    +++, strong suppression (> 4 log10/g faeces) or elimination of Enterobacteriaceae; ++, moderate suppression (approximately 3 log10/g 
    faeces); +, mild  suppression (< 2 log10/g faeces); -,  no significant effect; ND, not determined
    a A mean faecal concentration of 80 mg/kg was measured.
    b All persons became colonized with yeasts during treatment.
    c Colonization with yeasts, but not different from placebo group
    
    3.  COMMENTS

         Because ciprofloxacin has been shown to be a major metabolite of
    enrofloxacin in some food-producing animals, the Committee considered
    new information on the pharmacokinetics and effects of ciprofloxacin
    on human intestinal microflora. New studies of the microbiological
    effects of enrofloxacin and ciprofloxacin  in vitro were also
    evaluated by the Committee. All of these studies were conducted in
    accordance with appropriate standards for study protocol and conduct.

         The oral bioavailability of ciprofloxacin in humans is 63-69%,
    and this value is not significantly altered by administration with
    food. The main sites of absorption are the duodenum and jejunum. The
    recovery of high concentrations of ciprofloxacin in faeces is
    attributed primarily to secretion into the intestine.

         The Committee noted that fluoroquinolones as a class have a broad
    spectrum of activity against aerobic gram-negative bacteria. The
    primary human clinical use of the fluoroquinolones is for selective
    elimination of potential aerobic and facultative anaerobic pathogens
    from the gastrointestinal tract while preserving the predominant
    anaerobic bacterial flora. These properties of the drug are clinically
    useful in the treatment of travellers' diarrhoea, therapy for
    immunocompromised patients, selective decontamination prior to
    colorectal surgery, and therapy associated with burns and leukaemia.
    The Committee also noted that therapeutic oral doses of
    fluoroquinolones to humans have been shown to have no appreciable
    effect on the intestinal bacterial ecology and do not weaken the
    barrier effect. In addition, anaerobic bacteria such as
     Bifidobacterium, Bacteroides, Eubacterium, Fusobacterium, and
     Peptostreptococcus spp., the main components of the flora of the
    human gastrointestinal tract, are largely unaffected by these
    compounds. Finally, the Committee noted that, although  Escherichia 
     coli is extremely sensitive to fluoroquinolones in general, this
    species is a minor component of the flora in the gastrointestinal
    tract. These concepts guided the Committee in its evaluation and
    interpretation of the results of studies of the effects of
    enrofloxacin and ciprofloxacin on bacteria of human intestinal origin
     in vitro and  in vivo.

         The minimum concentration of enrofloxacin causing 50% inhibition
    (MIC50) was determined for 100 bacterial strains comprising 10
    isolates from 10 bacterial genera, many of which are typically found
    in the human gastrointestinal tract. These included  Enterococcus 
    spp.,  E. coli, Lactobacillus spp.,  Proteus spp.,  Bacteroides 
    spp.,  Bifidobacterium spp.,  Fusobacterium spp.,  Eubacterium 
    spp.,  Peptostreptococcus spp., and  Clostridium spp.  E. coli was
    found to be the most sensitive to enrofloxacin, with a mean MIC50
    value of 0.03 g/ml at an inoculum density of 107 colony forming units
    per ml. The mean MIC50 value for the 10 strains of the most
    sensitive relevant genus isolated from the human gastrointestinal
    tract was 0.125 g/ml for  Fusobacterium spp. at an inoculum density
    of 107 colony forming units per ml.

         A study of identical design was conducted to determine the
    MIC50 of ciprofloxacin against the same 100 bacterial strains. 
     E. coli was the most sensitive species, with a mean MIC50 value of
    0.016 g/ml at an inoculum density of 107 colony forming units per
    ml. The mean MIC50 value for the 10 strains of the most sensitive
    relevant genus isolated from the human gastrointestinal flora was
    0.031 g/ml for  Bifidobacterium spp. at an inoculum density of 107
    colony forming units per ml.

         A study to determine the microbiological activity of enrofloxacin
    metabolites against aerobic bacteria was reviewed by the Committee. In
    this study, the MIC50 values of enrofloxacin and nine of its
    metabolites were determined against 164 aerobic bacterial strains
    isolated from the human intestinal microflora. Enrofloxacin and
    ciprofloxacin were the most active compounds.  E. coli was the most
    sensitive species tested, the mean MIC50 values for enrofloxacin and
    ciprofloxacin against 33 strains of  E. coli being 0.030 and 0.015
    g/ml, respectively, at an inoculum density of 107 colony forming
    units per ml.

         The effect of pH on the MIC50 value of enrofloxacin against 36
    bacterial isolates representative of the human intestinal flora was
    considered. The pH levels tested (7.2, 6.2, and 5.2) encompassed the
    range of conditions likely to occur in the lower regions of the human
    intestinal tract, while avoiding extremes likely to inhibit bacterial
    growth  in vitro. The results were in agreement with those of
    previous studies with fluoroquinolones, which showed that
    antimicrobial activity decreased as the pH was lowered.

         The effect of enrofloxacin on the growth of 10 bacterial strains
    isolated from the human gastrointestinal tract was evaluated after
    incubation in a simple in-vitro model. Enrofloxacin was added to the
    test system at a concentration similar to the geometric mean MIC50
    previously determined for each microbial group and at 0.56 g/ml, the
    concentration estimated to occur in the intestine after consumption of
    residues. The viable counts of all 10 strains exposed to enrofloxacin
    increased during the 18-h incubation period. The growth of eight of
    the 10 strains was comparable to that of controls.

         Numerous reports on the effects of oral doses of ciprofloxacin on
    the intestinal flora of volunteers were available. The doses ranged
    from 50 mg twice daily to 750 mg three times daily. In general, the
    anaerobic microflora were, at most, mildly suppressed. The aerobic
    microflora were the most sensitive to all doses of ciprofloxacin
    tested.

         Three studies of the levels of ciprofloxacin in faecal material
    from volunteers who received oral doses of the compound enabled the
    Committee to obtain a more direct estimate of the concentration of the
    antimicrobial agent present in the colon than from the commonly used
    indirect approach of subtracting the value for bioavailability from 1.
    In these studies, volunteers received daily oral doses of
    ciprofloxacin ranging from 100 to 1000 mg for up to seven days. The

    concentrations of ciprofloxacin in faecal samples varied widely among
    individuals. Two of the studies resulted in mean data that were used
    to estimate that approximately 20% of an oral dose of ciprofloxacin is
    present in the colon, assuming a colonic content of 220 g. The
    Committee used this figure in establishing the upper limit of the ADI
    for the antimicrobial activity of enrofloxacin. This is in keeping
    with the decision taken at the forty-third meeting of the Committee to
    use data on excretion from studies with ciprofloxacin in humans for
    the purpose of calculating the upper limit of the ADI for the
    antimicrobial activity of enrofloxacin.

         The upper limit of the ADI based on the antimicrobial activity of
    enrofloxacin was calculated on the basis of the formula described on
    p. 12, as follows:


                            0.125 g/ga      220 g
         Upper limit   =                              
            of ADI          0.20b      1c      60 kg

                       =    2.3 g/kg bw


    a    For the purpose of this evaluation, the MIC50 value is the 
         mean MIC50 for enrofloxacin against the 10 strains of the
         sensitive relevant genus isolated from the human intestinal
         tract, in this case  Fusobacterium spp.

    b    The fraction of an oral dose available to act upon 
         microorganisms in the colon was determined on the basis of a
         study in volunteers in which about 20% of an oral dose of
         ciprofloxacin was present in the colon.

    c    A safety factor of 1 was used because extensive, relevant
         microbiological data were available.

    4.  EVALUATION

         The Committee noted that the antimicrobial activity of
    ciprofloxacin against the relevant human intestinal microflora was
    about four times greater than that of enrofloxacin and that consumers
    may be exposed to residues of ciprofloxacin in some species of 
    food-producing animals. The Committee considered that the
    approximately fourfold greater microbiological activity of
    ciprofloxacin should be taken into account in recommending MRLs for
    this metabolite.

         The forty-third Committee established a temporary ADI of 0-0.6
    g/kg bw on the basis of the limited summary data from microbiological
    tests on ciprofloxacin. The present Committee considered the
    toxicological data on enrofloxacin and the microbiological effects of
    enrofloxacin and ciprofloxacin and concluded that the  microbiological
    effects  in vitro were the most sensitive end-point on which to
    establish an ADI. Therefore, an ADI of 0-2 g/kg bw was established on
    the basis of data on the antimicrobial activity of enrofloxacin
    against 10 strains of the most sensitive relevant genus isolated from
    the human gastrointestinal tract. The Committee noted that this ADI
    provides an adequate margin of safety in relation to the NOEL of 1.2
    mg/kg bw per day for testicular toxicity in dogs described in the
    report of the forty-third Committee.

    5.  REFERENCES

    Bergan, T., Delin, C., Johansen, S., Kolstad, I.M., Nord, C.E. &
    Thorsteinsson, S.B. (1986) Pharmacokinetics of ciprofloxacin and
    effect of repeated dosage on salivary and faecal microflora.
    Antimicrob. Agents Chemother., 29, 298-302.

    Boisseau, J. (1993) Basis for the evaluation of the microbiological
    risks due to veterinary drug residues in food. Vet. Microbiol., 35,
    187-192.

    Brismar, B., Edlund, C., Malmborg, A.S. & Nord, C.E. (1990)
    Ciprofloxacin concentrations and impact of the colon microflora in
    patients undergoing colorectal surgery. Antimicrob. Agents Chemother.,
    34, 481-483.

    Brumfitt, W., Franklin, I., Grady, D., Hamilton-Miller, J.M.T. &
    Iliffe, A. (1984) Changes in the pharmacokinetics of ciprofloxacin and
    fecal flora during administration of a 7-day course to human
    volunteers. Antimicrob. Agents Chemother., 26, 757-761.

    Daikos, D.L., Kathpalia, S.B., Sharifi, R., Lolans, V.T. & Jackson,
    C.G. (1987) Comparison of ciprofloxacin and beta-lactam antibiotics in
    the treatment of urinary tract infections and alteration of fecal
    flora. Am. J. Med., 82, 290-294.

    Enzensberger, R., Shah, P.M. & Knothe, H. (1985) Impact of oral
    ciprofloxacin on the faecal flora of healthy volunteers. Infection,
    13, 273-275.

    Esposito, S., Barba, D., Galante, D., Gaeta, G.B. & Laghezza, O.
    (1987) Intestinal microflora changes induced by ciprofloxacin and
    treatment of portal-systemic encephalopathy. Drugs Exp. Clin. Res.,
    13, 641-646.

    Goldstein, E.J.C., Citron, D.M. & Corrado, M.L. (1987) Effect of
    inoculum size on  in vitro activity of norfloxacin against fecal
    anaerobic bacteria. Rationale for selective decontamination of the
    digestive tract. Am. J. Med., 82, 84-87.

    Hill, M.J. (1995) The normal gut bacterial flora. In: Hill, M.J., ed.,
    Role of Gut Bacteria in Human Toxicology and Pharmacology, London,
    Taylor & Francis, pp. 3-17.

    Holt, H.A., Lewis, D.A., White, L.O., Bastable, S.Y. & Reeves, D.S.
    (1986) Effect of oral ciprofloxacin on the faecal flora of healthy
    volunteers. Eur. J. Clin. Microbiol., 5, 201-205.

    Korten, V. & Murray, B. (1993) Impact of the fluoroquinolones on
    gastrointestinal flora. Drugs, 45 (Suppl. 3), 125-133.

    Lewin, C.S., Morrissey, I. & Smith, J.T. (1991) The mode of action of
    quinolones: The paradox in activity of low and high concentrations and
    activity in the anaerobic environment. Eur. J. Clin. Microbiol.
    Infect. Dis., 10, 240-248.

    Ljungberg, B., Nilsson-Ehic, L. & Nord, C.E. (1990) Influence of
    ciprofloxacin on the colonic microflora in young and elderly
    volunteers: No impact of the altered drug absorption. Scand. J.
    Infect. Dis., 22, 205-208.

    Marshall, A. (1996) Determination of the minimum inhibitory
    concentration (MIC) of enrofloxacin against 100 bacterial strains of
    human gut origin at three inoculum levels. Unpublished report No.
    DWS/014/96 from Don Whitley Scientific Ltd, Shipley, United Kingdom.
    Submitted to WHO by Bayer AG, Leverkusen, Germany.

    Midtvedt, T. (1990) The influence of quinolones on the faecal flora.
    Scand. J. Infect. Dis., 68, 14-18.

    Nord, C.E. (1993) The effect of antimicrobial agents on the ecology of
    the human intestinal microflora. Vet. Microbiol., 35, 193-197.

    Nord, C.E. (1995) Effect of quinolones on the human intestinal flora.
    Drugs, 49, 81-85.

    Pecquet, S., Ravoire, S. & Andremont, A. (1990) Faecal excretion of
    ciprofloxacin after a single oral dose and its effect on faecal
    bacteria in healthy volunteers. J. Antimicrob. Chemother., 26, 125-
    129.

    Pirro, F. (1996) Determination of the minimum inhibitory concentration
    (MIC) of enrofloxacin and 9 metabolites against 164 aerobic bacterial
    strains of human origin. Unpublished report No. 17164 from Bayer AG,
    Animal Health Research, Institute of Infectious Diseases and
    Antibacterial Chemotherapy, Germany. Submitted to WHO by Bayer AG,
    Leverkusen, Germany.

    Pridmore, A. (1996a) Determination of the minimum inhibitory
    concentration (MIC) of ciprofloxacin against 100 bacterial strains of
    human gut origin. Unpublished report No. DWS/021/96 from Don Whitley
    Scientific Ltd, Shipley, United Kingdom. Submitted to WHO by Bayer AG,
    Leverkusen, Germany.

    Pridmore, A. (1996b) An investigation into the effect of pH on the
    Minimum Inhibitory Concentration of enrofloxacin against bacterial
    isolates of human gut origin. Unpublished report No. DWS/022/96 from
    Don Whitley Scientific Ltd, Shipley, United Kingdom. Submitted to WHO
    by Bayer AG, Leverkusen, Germany.

    Rademaker, C.M., Hoepelman, I.M., Wolfhagen, M.J., Beumer, H. &
    Rosenberg-Arska, M. (1989) Results of a double-blind 
    placebo-controlled study using ciprofloxacin for prevention of
    traveler's diarrhea. Eur. J. Clin. Microbiol. Infect. Dis., 8, 
    690-694.

    Rosenberg-Arska, M., Dekker, A.W. & Verhoef, J. (1985) Ciprofloxacin
    for selective decontamination of the alimentary tract in patients with
    acute leukemia during remission induction treatment. The effect on
    fecal flora. J. Infect. Dis., 152, 104-107.

    van Saene, J.J.M., van Saene, H.K.F., Geitz, J.N., Tarko-Smit, N.J.PH.
    & Lerk, C.F. (1986) Quinolones and colonization resistance in human
    volunteers. Pharm. Weekbl. Sci. Ed., 8, 67-71.

    van Saene, H.K., Lemmens, S.E. & van Saene, J.J. (1988a) Gut
    decontamination by oral ofloxacin and ciprofloxacin in healthy
    volunteers. J. Antimicrob. Chemother., 22, 127-134.

    van Saene, J.J.M., van Saene, H.K.F., Geitz, J.N., Tarko-Smit, N.J.P.
    & Lerk, C.F. (1988b) Effects of ciprofloxacin on the intestinal flora.
    Rev. Infect. Dis., 10, 198.

    Scully, B.E., Jules, K., Chin, N.X. & Neu, H.C. (1987) Effect of
    ciprofloxacin on fecal flora of patients with cystic fibrosis and
    other patients treated with oral ciprofloxacin. Am. J. Med., 82, 
    336-338.

    Shah, P.M., Enzensberger, R., Glogau, O. & Knothe, H. (1987) Influence
    of oral ciprofloxacin or ofloxacin on fecal flora of health
    volunteers. Am. J. Med., 82, 333-335.

    Vollaard, E.J., Clasner, H.A.L. & Janssen, A.J.H.M. (1990) The
    contribution of Escherichia coli to microbial colonization resistance.
    J. Antimicrob. Chemother., 26, 411-418.

    de Vries-Hospers, H.G., Welling, G.W. & van der Waaij, D. (1987)
    Influence of quinolones on throat- and faecal flora of healthy
    volunteers. Pharm. Weekbl. Sci. Ed., 9, 41-44.

    Watson, P. (1996) Determination of the survival levels of human gut
    bacteria following exposure to enrofloxacin in simple  in vitro gut
    model. Unpublished report No. DWS/020/96 from Don Whitley Scientific
    Ltd, Shipley, United Kingdom. Submitted to WHO by Bayer AG,
    Leverkusen, Germany.

    Wolfson, J.S. & Hooper, D.C. (1991) Pharmacokinetics of quinolones:
    Newer aspects. Eur. J. Clin. Microbiol. Infect. Dis., 10, 267-274.
    


    See Also:
       Toxicological Abbreviations
       Enrofloxacin (WHO Food Additives Series 34)
       ENROFLOXACIN (JECFA Evaluation)